Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics. Such compositions are generally provided as imageable layers.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Because the presence of oxygen in the air can diminish the sensitivity of the radiation-sensitive composition, a hydrophilic oxygen-blocking protective layer (protective overcoat) is generally disposed over the radiation-sensitive composition. While it is known that good latent image keeping requires such as overcoat comprising a poly(vinyl alcohol) having a high saponification degree, if the saponification degree is too high, coating defects are too common because of the high dynamic contact angles formed with the poly(vinyl alcohol) coating solutions on the radiation-sensitive composition. The dynamic contact angle can be reduced by adding surfactants to the overcoat composition, but it is unpredictable about which surfactants will perform satisfactorily so the overcoat layer has the desired coefficient of friction in reference to interleaf papers used to separate printing plate precursors. Thus, the overcoat must be carefully designed to have a combination of properties.